Semiconductor pixel unit for sensing near-infrared light, optionally simultaneously with visible light, and a semiconductor sensor comprising same

ABSTRACT

A semiconductor pixel unit for sensing near-infrared light, and for optionally simultaneously sensing visible light. The pixel unit comprises a single substrate with a first semiconductor region and a second semiconductor region electrically separated by an insulating region, for example a buried oxide layer. The pixel unit is adapted for generating a lateral electrical field in the second region for facilitating transport of photoelectrons generated in the second region by near-infrared light passing through the first region and the insulating region.

FIELD OF THE INVENTION

The invention relates to the field of semiconductor structures fornear-infrared (NIR) light and optionally simultaneously sensing visiblelight. More in particular, the present invention relates to an imagingstructure (e.g. a pixel unit) capable of simultaneously capturingvisible light information (e.g. grayscale information or colorinformation) and near-infrared light information (e.g. representative ofdistance information also known as “time-of-flight” (TOF) information).

BACKGROUND OF THE INVENTION

Solid state silicon image sensors became ubiquitous in the recent years.The first high quality image sensors were fabricated in a CCD technologywhich is nowadays being more and more replaced by CMOS technology. Whiletypical CMOS based image sensor sensitivity is still lower than CCDbased sensors, the integration advantages that CMOS technology canprovide make it a technology of choice for today's image sensors.Besides the integration, CMOS technology provides building blocks foractive pixels which can extend the sensing capabilities of the imagesensors beyond just imaging such as e.g. distance measurement, night/fogvision using high speed shutter image sensors, etc.

For machine vision applications, despite increasing resolution,increasing dynamic range of image sensors and increasing computationalpower of CPUs, it is impossible for a machine to extract distanceinformation from a single 2D-image.

One solution is offered by so called stereo-vision systems. Intraditional stereo vision, two cameras, displaced horizontally from oneanother, are used to obtain two differing views of a same scene, in amanner similar to human binocular vision. By comparing these two images,the relative depth information can be calculated. However, this requirestwo cameras and a powerful processor.

Another solution is offered by so called “time-of-flight 3D cameras”,which provide depth information about the scene. Time-of-flight 3Dcameras typically use a near-infrared (NIR) (invisible to humans)modulated or pulsed light source to illuminate the scene, and thereflected near-infrared light is detected by high bandwidthtime-of-flight pixels. The time it takes for a light signal to travelfrom the light source (emitter) to the object and back is proportionalto the distance between the emitter and the object. This time delaybetween the emitted signal and the detected signal, also called“time-of-flight (TOF)” or “round-trip time (RTT)”, is usually estimatedby frequency domain techniques (e.g. demodulation) or time domaintechniques (e.g. correlation). Such “time-of-flight pixels” andcorresponding processing circuitry exist, and are known in the art.

Although it is possible to get an image from a (pure) time-of-flightcamera, the quality of such an image would be much inferior to the imagequality of a typical image sensor as used e.g. in digital cameras. Oneof the shortcomings is image resolution. Typical time-of-flight pixelsare typically much larger than typical image sensor pixels to increasethe sensitivity. Another problem is that image sensors andtime-of-flight cameras set different requirements towards the optics. Atime-of-flight camera typically requires only a narrow band ofnear-infrared light to reduce the shot noise and early saturation (e.g.due to sunlight), whereas an image sensor requires only visible light.

In order to get both image information (e.g. intensity and/or colorinformation) and distance information, some prior art systems combine animage camera and a 3D-camera, by implementing them as two separatesystems. However this requires a high system cost.

US2013/0234029 discloses an image sensor for two-dimensional andthree-dimensional image capture, comprising visible light photodetectorsembedded in a first substrate, and TOF photodetectors embedded in asecond substrate, which two substrates are combined by a bondingtechnique to form a single chip. However, the cost of such a sensor isrelatively high.

US2007051876A1 discloses an imager in which a visible image and aninfrared image can be independently and simultaneously obtained. Thesolution proposed requires two substrates oriented perpendicular to eachother, and a mirror oriented at 45° with respect to these substrates.

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide agood semiconductor pixel unit for sensing modulated near-infrared light.The modulated near-infrared light may be indicative or representative ofdistance information.

It is an object of embodiments of the present invention to provide asemiconductor device and/or pixel unit that has an improved demodulationcontrast for the near-infrared light, e.g. a higher (de)modulationcontrast for a given (de)modulation frequency, or a same demodulationcontrast for a higher (de)modulation frequency.

It is an object of embodiments of the present invention to provide agood semiconductor pixel unit that combines a sensor for sensing visiblelight and a sensor for sensing modulated near-infrared light. Themodulated near-infrared light may be indicative or representative ofdistance information.

It is an object of particular embodiments of the present invention toprovide such a pixel unit capable of simultaneously capturing saidvisible light and said modulated near-infrared light.

It is an object of embodiments of the present invention to provide agood semiconductor device that combines a two dimensional image sensorwith a three dimensional time-of-flight (TOF) distance sensor.

It is an object of embodiments of the present invention to provide sucha semiconductor device and such a pixel unit that can be manufactured ina cost efficient manner.

It is an object of embodiments of the present invention to provide sucha semiconductor device that is capable of simultaneously capturingvisible light information and near-infrared light information, both at agood resolution.

It is an object of particular embodiments of the present invention toprovide such a semiconductor device and/or pixel unit that have animproved quality (e.g. increased sensitivity, e.g. improved demodulationcontrast) of the TOF-information and/or an improved quality (e.g.increased spatial resolution) for the visible image.

This objective is accomplished by a device according to embodiments ofthe present invention.

According to a first aspect, the present invention provides asemiconductor pixel unit adapted for simultaneously sensing visiblelight and near-infrared light, the semiconductor pixel unit comprising:a single semiconductor substrate comprising a first semiconductor regionand a second semiconductor region electrically separated from the firstsemiconductor region by means of an insulating region, the firstsemiconductor region comprising at least one visible light detector fordetecting photoelectrons generated in the first semiconductor region bythe visible light, means for generating a lateral electrical field in aregion underneath the first semiconductor region, the electrical fieldbeing adapted for facilitating or promoting transport of photoelectronsgenerated in the second semiconductor region by the near-infrared lightafter passing through the first semiconductor region and through theinsulating region, the second semiconductor region comprising at leastone near infrared light detector located at the surface of thesemiconductor substrate for detecting the photoelectrons generated inthe second semiconductor region by the near infrared light.

It is an advantage of embodiments of the present invention that bothvisible light information and near-infrared (NIR) light information canbe captured by a single structure. Moreover, that it can be capturedsimultaneously, which may be important when capturing moving objects, soas to provide consistent data.

It is a major advantage that the first and second semiconductor regionare separated by an electrical insulating region, because in that wayalmost no visible light information can enter (e.g. drift to) the secondsemiconductor region, which significantly improves Signal to Noise Ratio(SNR) of the distance information. The second semiconductor region isalso referred to herein as the TOF detection region.

It is a further advantage of the isolated block that it can help toreduce power consumption.

Embodiments of the present invention combine the functionality of apicture sensor and a time-of-flight (TOF) sensor with a minimum loss ofperformance and/or quality.

The first semiconductor region may comprise one or more sub-regions,which may, but need not be electrically separated from each other.

It is an advantage that, by locating the second semiconductor regionunderneath the first semiconductor region embedded in the samesubstrate, the available silicon area is (roughly stated) used twice fordifferent functionality (like a building having two floors instead ofonly one).

It is an advantage of embodiments of the present invention that thevisible light (e.g. color) information and near-infrared light (e.g.TOF) information originating from a same object will be captured in aconsistent manner, because the infrared sensing area (secondsemiconductor region) of a pixel unit is located underneath the visiblelight (e.g. color) sensing area (first semiconductor region), and thusare necessarily aligned. In other words, substantially the same opticalpath is followed by the visible light components and infrared lightcomponents originating from the same object, but the visible light willpenetrate less deep into the substrate than the near-infrared light.

It is an advantage of embodiments of the present invention that the atleast one visible light detector is arranged for detecting chargecarriers generated in the first semiconductor region of the substrate,in response to visible light incident on the substrate.

It is an advantage of embodiments of the present invention that the atleast one near-infrared light detector is arranged for detecting chargecarriers generated in the second semiconductor region of the substrate,in response to near-infrared light incident on the substrate.

It is an advantage of embodiments of the present invention that alateral electrical field is present in at least part of the secondsemiconductor region, in particular below the first semiconductorregions, to move the photoelectrons generated in the secondsemiconductor region towards at least one corresponding detector(called: NIR detector). Thanks to the presence of the electrical field,this transfer occurs fast, so that the detected, e.g. measurednear-infrared light detection can be used for distance measurement. Forsuch applications, the semiconductor pixel unit is adapted for detectingmodulated or pulsed near-infrared light emitted by a near infrared lightemitter, e.g. a LED, typically located near the pixel unit (e.g. at adistance less than 5 cm).

The insulating region may have a horizontal segment (e.g. parallel tothe substrate surface), and a vertical or inclined segment (e.g.perpendicular to the substrate surface) for separating the first and thesecond semiconductor region.

It is advantageous that the vertical or inclined segments extend fromthe top to the horizontal segment. Alternatively the insulating regionmay have a bathtub-shape. Such shapes can be generated e.g. usingstandard SOI technology, optionally in combination with the formation oftrenches (e.g. so called Deep Trench Isolation).

It is an advantage that the insulating region has a predefined thicknessand is chosen from a material (e.g. SiO₂) which is at least partlytransparent for said near-infrared light. Preferably the transmissioncoefficient of the near-infrared light of interest (e.g. in the range ofabout 820 to 880 nm) is at least 0.6.

It is an advantage that the first and the second semiconductor regionsare build in a single substrate, which can be achieved for example byusing silicon-on-insulator (SOI) technology, rather than having to usetwo individual substrates which need to be produced separately, and needto be bonded together. This offers advantages in mechanical stability,but also in price.

It is an advantage that the first semiconductor region (with the one ormore image subpixels, e.g. color subpixels) is located on top of thesecond semiconductor region (“NIR region”), because in this way the areaoccupied by both regions can be increased, e.g. maximized. This allows ahigh resolution of the image pixels, (for example at least 1Megapixels), and allows relatively large pixels for detecting the NIRlight.

It is an advantage of embodiments according to the present inventionthat the resolution of the “NIR pixels” may be in the same order as theresolution of the visible pixels, or can be chosen much smaller than theimage resolution, depending on the application, depending on how manyvisible light detectors are present in the first semiconductor region.By increasing the size of the NIR region, the sensitivity of thenear-infrared detection can be increased, thus a high sensitivity can beobtained without compromising the resolution of the visible pixels.

It is an advantage of embodiments according to the present inventionthat the “NIR pixels” do not require a large detector, e.g. photodiode,thus, they can be made smaller than image pixel sensors without asignificant loss of sensitivity.

It is an advantage that the detectors may have separate readoutcircuitry, or that one or more readout-circuitry can be shared amongstdifferent detectors.

The thickness of the first semiconductor layer is preferably in therange of 3 to 5 micron. This offers the advantage that a majority of thevisible light is absorbed by the first semiconductor layer beforereaching the second semiconductor region, hence the disturbance for theNIR detection is reduced.

Such a semiconductor pixel unit is ideally suited for use in asingle-chip implementation in the field of 3D-camera systems.

The first semiconductor region and the second semiconductor region maycontain mainly silicon, or mainly Germanium or SiGe or InGaAs orSi-compounds or Ge-compounds, or any other suitable semiconductormaterial.

The second semiconductor material may be the same material as the firstsemiconductor material, or may be a different material.

In a specific embodiment, the first and second semiconductor regioncomprises mainly (e.g. at least 95%) silicon, and the insulating layercomprise mainly (e.g. at least 95%) silicon-oxide or silicon nitride.The silicon-oxide may be deposited or grown on the second semiconductorlayer. The silicon nitride may be deposited on the second semiconductorlayer.

The projected area (in a plane parallel to the substrate surface) of theNIR detection region (where the photoelectrons are generated due to NIRlight) may be about equal (+/−20%) to the sum of the projected areas ofthe image pixels (where the photoelectrons are generated by visiblelight).

According to a second aspect, the present invention provides asemiconductor pixel unit adapted for sensing near-infrared light, thesemiconductor pixel unit comprising: a single semiconductor substratecomprising a first semiconductor region and a second semiconductorregion electrically separated from the first semiconductor region bymeans of an insulating region; means for generating a lateral electricalfield in a region underneath the first semiconductor region, theelectrical field being adapted for promoting transport of photoelectronsin the second semiconductor region generated by the near-infrared lightafter passing through the first semiconductor region and through theinsulating region; the second semiconductor region comprising at leastone near infrared light detector located at the surface of thesemiconductor substrate for detecting the photoelectrons generated inthe second semiconductor region by the near infrared light.

The pixel unit according to the second aspect is very similar to thepixel unit of the first aspect, except that the pixel unit of the secondaspect does not have provisions for sensing visible light, only forsensing NIR light.

The same advantages as were mentioned for the pixel unit according tothe first aspect (except of course those related to visible light, e.g.resolution of the visible light pixels), are also valid for the pixelunit of the second aspect. In particular, the quality of the NIR signalis nearly the same for both pixel units, (inter alia) because theabsorption of visible light by the material of the first semiconductorregion remains substantially the same, even though the visible light isnot measured and/or readout, thus the quality of the NIR signal is alsosubstantially the same.

Unless explicitly mentioned otherwise, the embodiments describedfurther, can be embodiments of the pixel unit according to the firstaspect and/or embodiments of the pixel unit according to the secondaspect.

In an embodiment, the pixel unit further comprises means for negativelybiasing the first semiconductor region with respect to the secondsemiconductor region to affect the energy bands near the surface of thesecond semiconductor region over at least a portion of the insulationregion, so that the photoelectrons generated in the second semiconductorregion by the near-infrared light are pushed away from the insulatingregion.

In this embodiment the first semiconductor region is used as anelectrode located on top of the insulating region, the electrode beingtransparent to at least NIR light, optionally also to visible light. Theelectrode may comprise or consist of polysilicon.

The inventors surprisingly found that the performance (e.g. in terms ofdemodulation contrast) of the NIR light detection can be drasticallyimproved by applying such a bias voltage. For example, for a given(de)modulation frequency, the demodulation contrast can be increased. Orstated in other words, for a given demodulation contrast, the(de)modulation frequency can be increased. This translates into a moreaccurate distance measurement (or depth measurement), and/or in a higherresolution NIR image (in case of a pixel array).

In an embodiment, the insulating region between the first semiconductorregion and the second semiconductor region comprises a buried oxidelayer.

It is an advantage that a substrate with a buried oxide layer can beprovided by the known SOI (silicon-on-insulator) technology, which is amature technology.

The buried oxide layer may have a thickness of about 1.0 micron. Theburied oxide layer may have a bath-tub shape, or a substantiallyhorizontal shape (i.e. parallel to the substrate surface).

It is an advantage of using an oxide between the first and secondsemiconductor layer in that it does not allow or at least reducestransport of photoelectrons generated in the first region to travel tothe second region (e.g. via diffusion), and vice versa. In this way theaccuracy of both the visible light detection and of the NIR detection isincreased.

It is an advantage of using an oxide between the first and secondsemiconductor layer in that it electrically decouples the means ofcreation of deep lateral electrical field drift region in the secondsemiconductor region from the first semiconductor region containingvisible pixels which can be built using a well established CIS (CMOSimage sensor) technology.

The buried oxide layer may have a bath-tub shape.

The buried oxide layer may have a shape comprising a major portionparallel to the substrate surface.

Preferably the buried oxide layer is a single layer, not a multilayer.

The buried oxide may have a flat shape, the first semiconductor region101 residing on top of the buried oxide, the second semiconductor regionbeing located underneath. The first semiconductor region 101 can e.g. bemade of polysilicon or monocrystalline silicon.

The insulating region between the first semiconductor region and thesecond semiconductor region may further comprise at least one insulatingtrench.

It is an advantage that insulating trenches extend in a directionperpendicular to the surface, because this occupies less space than e.g.inclined edges. The trenches may be so called “DTI”-trenches (“DeepTrench Isolation”). Such trenches can be conveniently combined with ahorizontal segment formed by SOI-technology to isolate a first (upper)region from a second (lower) region.

It is an advantage that such trenches can serve as an electrical barrierbetween the NIR-detectors and the visible light detectors, thus avoidingthat photoelectrons generated in the first semiconductor region aredetected by the NIR detectors, and that photoelectrons generated in thesecond semiconductor region are detected by image detectors.

Preferably the first semiconductor region is laterally surrounded by oneor more insulating trenches. The trenches may extend from thesemiconductor surface to the insulating layer.

It is an advantage of embodiments wherein the at least one trenchextends from the surface of the substrate to the buried oxide layer,thereby completely electrically separating the first and the secondsemiconductor region. Although complete electrical separation canincrease the accuracy and reduce cross-talk, complete electricalseparation is not absolutely necessary for the device to work.

Preferably, the first semiconductor region is electrically completelyisolated from the second semiconductor region by an oxide layer (e.g. inthe vertical direction by a burned oxide layer, and in the horizontaldirection by deep trenches).

In an embodiment, the first semiconductor region comprises at least twovisible light detectors laterally separated by at least one insulatingtrench.

Although not strictly required for functioning, it is an advantage ofembodiments of the present invention that individual visible lightdetectors present in the first semiconductor region are at least partly(with STI, shallow trenches) or completely (with DTI, deep trenches)isolated from each other by means of trenches, because of reducedcross-talk. Hence, such trenches offer an increased image quality. Ifthe bathtub DTI are omitted it is an advantage of the insulating trenchthat it prevents a lateral electrical current flow (caused by the meansfor deep lateral electrical field generation) through the firstsemiconductor region; such a lateral electrical current through thefirst region, if present, can cause disruption in operation of thevisible pixels, means for the deep lateral electrical field generationor excessive power consumption.

In an embodiment, the second semiconductor region has a non-constantdoping profile as a means for generating said lateral electrical field.

It is an advantage of such embodiment that the electrical field fortransporting the photoelectrons in the second semiconductor region isformed in a passive manner (without having to apply power). Thenon-constant doping profile should be monotonically decreasing to one orto both sides of the pixel unit, and should not have a local minimum.

In some embodiments the doping profile may have a local maximum near themiddle of the first semiconductor region (in lateral direction). This isadvantageous because the time to travel from a random point in thesecond semiconductor region where the photoelectron is generated to oneof the detector regions, has a lower statistical spread than would bethe case if the maximum doping profile is located near one lateral endof the first semiconductor region. This allows an improved accuracy ofTOF-detection.

In other embodiments the doping profile may have a local maximum nearone end of the first semiconductor region (in lateral direction). Thiscan be advantageous in that only a single NIR-detector is required,which may save space and circuitry.

In an embodiment, the second semiconductor region has an intrinsicdoping level, and the semiconductor pixel unit has a first detectorelement being an n-doped region and a second detector element being ap-doped region, so as to form a P-I-N structure.

It is an advantage of this embodiment that it has a reduced, e.g.negligible power consumption of the sensor. The NIR-signal can bereadout by using an external demodulating circuit.

It is an advantage of this embodiment that the NIR-signal can bedemodulated by modulating the reverse bias of the P-I-N structure.

In an embodiment, the second semiconductor region is a lowly p-dopedregion, and the semiconductor pixel unit has a first detector elementand a second detector element, and further comprises a first p-dopedcontact region arranged in the second semiconductor region adjacent thefirst detector element, and a second p-doped contact region arranged inthe second semiconductor region adjacent the second detector element,the first and the second contact regions allowing a voltage to beapplied for creating said lateral drift field.

With “lowly doped” is meant having a doping level of in the order of10¹²/cm³.

An advantage of this embodiment is that it provides an easy way toperform a demodulation in the second semiconductor region by applying amodulated voltage over said first and second contact regions, (e.g. asine wave or block wave etc.) which modulates the current flowingthrough the second semiconductor region.

In an embodiment, the semiconductor pixel unit further comprises atleast one readout circuit for reading out data of the visible lightdetector.

The readout circuit may have for example a classical 3T or 4Tarchitecture. Each image detector, e.g. photodetector, may have itsproper readout circuit. Alternatively one readout circuitry can beshared (time-multiplexed) by more than one image detector.

In an embodiment, the semiconductor pixel unit further comprises atleast one readout circuit for reading out data of the near infraredlight detector.

The readout circuit may have for example a classical 3T or 4Tarchitecture. Each NIR-detector, e.g. photodiode may have its properreadout circuit, or readout circuitry can be shared (time-multiplexed)by more than one NIR detector, or by NIR detectors and image detectors,or the readout circuitry may also be external.

In an embodiment, the semiconductor pixel unit further comprises ademodulator, (e.g. a mixer), having a first input connected to the NIRdetector and a second input for receiving a modulation waveform from amodulator, and having an output for providing a demodulated NIR signalto a distance determination circuit.

It is an advantage of using heterodyning as a demodulation technique,because it is relatively simple and reliable. By such demodulation, theDC-offset caused by e.g. sunlight may be automatically removed.

It is an advantage of using a demodulating circuit outside of an arrayto maximize the photo sensitive silicon area for applications whichrequire distance information with low spatial resolution.

In an embodiment, the semiconductor pixel unit further comprises saiddemodulator adapted for operating at a demodulation frequency of atleast 10 MHz, for example at least 20 MHz, for example at least 40 MHz,for example at least 80 MHz, for example at least 100 MHz, for exampleat least 150 MHz, for example at least 200 MHz, and a predefined DCvoltage source for providing the bias voltage, the bias voltage beingchosen such that the demodulation contrast of the pixel unit is at least50%.

The semiconductor pixel unit may be part of a semiconductor sensor,further comprising said modulator adapted for operating at a frequencyof at least 10 MHz, for example at least 20 MHz, for example at least 40MHz, for example at least 80 MHz, for example at least 100 MHz, forexample at least 150 MHz, for example at least 200 MHz.

In an embodiment, the semiconductor pixel unit further comprises atleast one microlens arranged on top of said at least one visible lightdetector.

It is an advantage of using microlenses that they concentrate (focus)incident light on the metallization-free photo sensitive silicon area.This improves the visible and NIR pixel sensitivity and reducescross-talk.

Although not essential for the working of the device, microlenses candecrease cross-talk, and thus improve image quality, and increase theaccuracy of TOF detection.

In an embodiment, the semiconductor pixel unit further comprises atleast one color filter arranged on top of said at least one visiblelight detector, the color filter having a characteristic for passingvisible light in a first predefined band of spectrum, and for blockingor at least attenuating visible light in a second predefined band of thespectrum, and for passing a predefined band of near infrared light.

The predefined band of near-infrared light may e.g. be a band from 800to 900 nm, or a band from 900 nm to 950 nm, or any other band in theNIR-spectrum.

Color filters can be used to make the image sensor sensitive forparticular colors, but the use of color filters are not essential forthe present invention, because it is also possible to make a grayscaleimage sensor.

Preferably multiple color filters are present, passing and blockingdifferent bands in the visible spectrum, such as a red, green and blueband, but other color filters can also be used. By doing so, the imagesensor can detect different colors of a scene, and effectively is acolor sensor.

In an embodiment, the semiconductor pixel unit further comprises anoptical filter having a characteristic for passing light in a first bandfrom about 380 to about 750 nm, and for blocking or at least attenuatinglight in a second band from about 750 nm to a predefined first frequencyand for passing near-infrared light in a third band from the predefinedfirst frequency to a predefined second frequency, whereby the predefinedfirst frequency is a value in the range of 750 nm to a wavelength belowthe maximum wavelength of semiconductor sensitivity (e.g. about 1100 nmfor silicium), and the predefined second frequency is a value in therange from the predefined first frequency to the maximum wavelength ofsemiconductor sensitivity.

In an example, the second band is a band from about 750 nm to about 800nm, and the third band is a band from about 800 nm to about 900 nm. Inanother example, the second band is a band from about 790 nm to about900 nm, and the third band is a band from about 900 nm to about 950 nm,but other second and third bands are also possible.

It is an advantage of using such a filter, when used for time-of-flightapplications, because it removes most of the disturbance signals,thereby improving the accuracy of TOF-information.

According to a third aspect, the present invention provides asemiconductor sensor comprising: a sensor array comprising a pluralityof semiconductor pixel units according to the first aspect; a lightsource for emitting a modulated or pulsed near-infrared light to bereflected against objects in a scene to be captured; a distancedetermination circuit for deriving distance information based on theemitted near-infrared light and the measured reflected near-infraredlight.

The light source may be a LED or laser, e.g. a laser diode.

The means for deriving distance information and/or color information maycomprise a processing unit adapted for performing calculations, such ase.g. color conversion, using algorithms known in the art.

In an embodiment, the semiconductor sensor further comprises: saidmodulator, adapted for operating at a predefined frequency of at least20 MHz, and said demodulator adapted for operating at the samepredefined frequency, and a predefined DC voltage source for providingthe bias voltage between the first and second semiconductor region, thebias voltage preferably being chosen such that the demodulation contrastof the pixel unit is at least 50%.

According to a fourth aspect, the present invention provides a digitalcamera comprising the semiconductor sensor of the second aspect.Preferably, the digital camera comprises only a single semiconductorsubstrate (not two) and does not contain a mirror (e.g. a mirror asdescribed in US2007051876A1).

In an embodiment, the digital camera further comprises a light sourceand circuitry for transmitting modulated infrared or near-infraredlight, and circuitry for demodulating the received near-infrared lightand for converting it into distance information.

Particular and preferred aspects of the invention are set out in theaccompanying independent and dependent claims. Features from thedependent claims may be combined with features of the independent claimsand with features of other dependent claims as appropriate and notmerely as explicitly set out in the claims.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic drawing of a basic structure of asemiconductor pixel unit according to an embodiment of the presentinvention for simultaneously sensing visible light and NIR-light.

FIGS. 2(a) and 2(b) show a variant of the structure of FIG. 1, whereinthe second (lower) semiconductor region has a non-constant dopingprofile as a means for creating a lateral electrical field.

FIG. 3 shows another variant of the structure of FIG. 1, wherein thesecond (lower) semiconductor region is substantially un-doped (i.e.intrinsic), and wherein the first semiconductor region contains a firstdetection region being an n-doped region, and a second detection regionbeing a p-doped region, so as to form a “P-I-N” structure.

FIGS. 4(a) and 4(b) show another variant of the structure of FIG. 1,wherein the second (lower) semiconductor region is a lowly p-dopedregion, and wherein each of the first and second detection regioncomprises a p-doped region adjacent a n-doped region.

FIG. 5 shows a typical spectrum of sunlight, and a transmissioncharacteristic of silicon for visible light, and the product of both,showing a typical sunlight spectrum reaching the second (lower)semiconductor region.

FIG. 6 shows a top view of a possible arrangement of a sensor arraycomprising multiple pixel units, whereby a plurality of NIR detectionregions are interconnected, according to an embodiment of the presentinvention.

FIG. 7 shows a top view of another possible arrangement of a sensorarray comprising multiple pixel units according to an embodiment of thepresent invention.

FIG. 8 shows a particular embodiment of a semiconductor pixel unitaccording to the present invention, further comprising micro-lenses andspectral filters.

FIGS. 9(a)-9(e) show examples of suitable filters as may be used in theembodiment of FIG. 8.

FIG. 10 shows (dotted line) the optical transmission characteristic of asilicon film having a thickness of 5 micron resp. of 3.5 micron forincident radiation having a wavelength in the range of about 400 toabout 1000 nm.

FIG. 11 shows the spectrum of FIG. 5 in combination with the “short passand bandpass” filter of FIG. 9(a).

FIG. 12 shows a manner of pushing photoelectrons generated in the secondsemiconductor away from the silicon-oxide interface, by using the firstsemiconductor region as a bias electrode, and by negatively biasing thefirst semiconductor region with respect to the second semiconductorregion.

FIG. 13(a) shows a conduction band diagram for an electrically neutraloxide, located between two highly doped p+ regions, which in turn arelocated between two highly doped n+ regions, when a voltage is appliedover said p+ regions.

FIG. 13(b) shows a conduction band diagram of the structure of FIG.13(a), for a positively charged oxide (e.g. an oxide comprising positivecharge carriers trapped therein).

FIG. 14 shows data of an experimental performance measurement on a firstexperimental device having a structure similar to the structure shown inFIG. 4, and having a bias voltage applied over the oxide as shown inFIG. 12, showing “demodulation contrast” versus “modulation frequency”,for bias voltages applied over the oxide ranging from 0V to −4V, and fora modulation frequency in the range of 10 MHz to about 400 MHz.

FIG. 15 shows data of an experimental performance measurement on asecond experimental device having a structure similar to the structureshown in FIG. 4 (but without the first semiconductor region on top andhence no bias voltage applied over the oxide), showing “demodulationcontrast” versus “modulation frequency”, and for a modulation frequencyin the range of 300 kHz to 60 MHz.

FIG. 16 is a qualitative graph for indicating a typical improvement of“demodulation contrast” of a pixel according to the present invention,by applying a bias voltage between the first and second semiconductorregion. (the values on either axis being exemplary values).

FIG. 17 is a variant of FIG. 12 for illustrating embodiments of thepresent invention not having pixels for sensing visible light.

The drawings are only schematic and are non-limiting. In the drawings,the size of some of the elements may be exaggerated and not drawn onscale for illustrative purposes.

Any reference signs in the claims shall not be construed as limiting thescope. In the different drawings, the same reference signs refer to thesame or analogous elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. The dimensions and the relative dimensions do notcorrespond to actual reductions to practice of the invention.

Furthermore, the terms first, second and the like in the description andin the claims, are used for distinguishing between similar elements andnot necessarily for describing a sequence, either temporally, spatially,in ranking or in any other manner. It is to be understood that the termsso used are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other sequences than described or illustrated herein.

Moreover, the terms top, under and the like in the description and theclaims are used for descriptive purposes and not necessarily fordescribing relative positions. It is to be understood that the terms soused are interchangeable under appropriate circumstances and that theembodiments of the invention described herein are capable of operationin other orientations than described or illustrated herein.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. It is thus tobe interpreted as specifying the presence of the stated features,integers, steps or components as referred to, but does not preclude thepresence or addition of one or more other features, integers, steps orcomponents, or groups thereof. Thus, the scope of the expression “adevice comprising means A and B” should not be limited to devicesconsisting only of components A and B. It means that with respect to thepresent invention, the only relevant components of the device are A andB.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly it should be appreciated that in the description of exemplaryembodiments of the invention, various features of the invention aresometimes grouped together in a single embodiment, figure, ordescription thereof for the purpose of streamlining the disclosure andaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the detailed description are hereby expressly incorporatedinto this detailed description, with each claim standing on its own as aseparate embodiment of this invention.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art. For example, in the following claims, any of theclaimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

Where in the present invention reference is made to “visible light”, theportion of the electromagnetic spectrum that is visible (can bedetected) by the human eye is meant. A typical human eye is sensitive towavelengths of about 390 nm to about 700 nm. Different wavelengths aresensed by the human eye as different colors, for example 450 to 495 nmis sensed as “blue”, 495 to 570 nm is sensed as “green”, and 620 to 750nm is sensed as “red”. Therefore, the corresponding data is referred toherein as “color data” or “color information” or generally as “imageinformation”.

Where in the present invention reference is made to “near-infrared”light, (abbreviated as NIR), reference is made to electromagnetic waveshaving a wavelength in the range of about 750 nm to about 1150 nm, or asubset thereof, such as for example a range of about 800 nm to about 900nm.

Where in the present invention reference is made to “visible subpixel”or “color subpixel” or “visible pixel” (for example “red subpixel” or“red pixel”) or “visible light sensor”, reference is made to that partof the semiconductor device comprising the first (upper) semiconductorregion (typically 5 micron deep) where photoelectrons are generated byincident visible light. This area comprises a “sensor element” (ordetection element) for measuring said photoelectrons. This sensorelement is referred to herein as “image photodetector” or “visible lightdetector”.

Where in the present invention reference is made to “NIR subpixel” or“NIR pixel” or “near infrared sensor”, reference is made to that part ofthe semiconductor structure comprising the second (lower) semiconductorregion where photoelectrons are generated by incident near infraredlight, the main part of which is located below the insulating layer, butit also includes the space between individual insulation layers, wherea.o. electrical contact elements may be provided for generating anelectrical field in the second semiconductor region. This area alsocomprises a “sensor element” for measuring said photoelectrons. Thissensor element is referred to herein as “near infrared detector” orsometimes also as “time of flight detector”.

Where in the present invention the expression “the optical filter allowspassage of a given frequency range” is used, what is meant is that thetransmission coefficient in that frequency range is relatively high,e.g. at least 0.6.

Where in the present invention the expression “the optical filter blocksor attenuates a given frequency range”, what is meant is that thetransmission coefficient in that frequency range is relatively low, e.g.at most 0.4.

In the present invention the terms “distance information” and “depthinformation” are used as synonyms. The term “time of flight information”(abbreviated as “TOF”) corresponds to the time required for light totravel over this distance.

Although it is contemplated that the present invention works for manyseveral semiconductor materials and/or several materials of theinsulating layer, the invention will be explained for silicon as thefirst & second semiconductor material and silicon-oxide as the materialof the insulating layer. But the invention is not limited to only thesematerials.

As illustrated in FIG. 1 to FIG. 16, the present invention proposes asemiconductor pixel unit capable of capturing both visible light (e.g.intensity information or color information) and near-infrared (NIR)light. Later, in FIG. 17, variants of this structure will be discussednot having provisions for capturing visible light, only for capturingNIR light.

In all cases, the NIR light may comprise a reflected near-infrared lightcomponent originating from a pulsed or modulated light source (emitter),typically located in the vicinity of the semiconductor pixel unit orsensor array comprising same. By determining the time difference orphase difference between the emitted light and the reflected light orbetween corresponding electrical signals, “time-of-flight” (TOF)information or “distance” information can be derived. An array of suchpixel units is thus capable of capturing two-dimensional imageinformation (e.g. color data), as well as three-dimensional information(e.g. depth information for each pixel position of the array).

As mentioned in the background section, separate devices for eithermeasuring color information or depth information are known in the art,and it is not easy to combine these two functions in a single integrateddevice, a.o. because TOF sensors and image sensors impose differentrequirements for the optical system: (1) image sensors should detectonly visible light and not near-infrared light, whereas TOF sensorsshould detect only near-infrared light and not visible light; (2) imagedetectors should have a relatively small size for providing a highspatial resolution (e.g. at least 1 Megapixels), whereas TOF detectorsshould have a relatively large size (typically in the order of 15micron×15 micron) for providing a high sensitivity. Hence, providing apixel unit where color detectors and NIR detectors are interleaved, forexample as four square subpixels of equal size, whereby three detectorsare arranged for measuring color information (e.g. red, green and blue),and one detector is arranged for measuring NIR information, does notprovide a good solution because 75% of the NIR power would be lost.Drastically increasing the size of the NIR detector while decreasing thesize of the color detectors is not a good solution either, because thequality of the visual information would severely suffer. It is thereforea challenge to propose a good pixel unit capable of detecting both colorinformation and NIR information.

FIG. 1 is a simplified schematic cross sectional drawing of the basicstructure of a semiconductor pixel unit 120 according to embodiments ofthe present invention. It shows a single semiconductor substratecomprising a first semiconductor region 101 for detecting visible light(e.g. color information). The first semiconductor region may contain asingle region, or multiple individual regions further referred to hereinas visible pixels or color subpixels. The first semiconductor region 101is arranged on top of, and is electrically isolated from a secondsemiconductor region 104 by means of an insulating layer 102. In theexample shown, the insulating layer 102 is a combination of a buriedoxide layer (portion that is substantially parallel to the surface),abbreviated as “BOX”, and upright regions (e.g. portions which aresubstantially perpendicular to the surface). The horizontal part of thisinsulating layer can be manufactured e.g. using SOI (Silicon OnInsulator) technology, the upright regions can be manufactured e.g. bythe formation of trenches, e.g. deep trenches, known as “DTI”. The areasbetween horizontal BOX parts of different pixel units may be formed forexample by making use of the so called “handle wafer contact” feature,which is known in the field of SOI-technology.

The insulating layer 102 can be also implemented as a flat layer,parallel to the surface, with the first (upper) semiconductor region 101above it, and the second (lower) semiconductor region 104 beneath it.

The present invention proposes a semiconductor pixel unit 120 and asemiconductor sensor 100 comprising same for combining near-infrareddetectors (e.g. fast near-infrared light sensitive pixels) and visiblelight detectors (e.g. visible range light sensitive pixels) on a singlesubstrate, in a way that avoids the sensitivity-versus-resolutiontradeoff mentioned above. This is achieved by splitting the sensitivesilicon volume (e.g. the upper 20 micron near the surface) into tworegions 101 (upper region), 104 (lower region) separated by aninsulating medium 102 which is substantially transparent tonear-infrared light (e.g. has a transmission coefficient of at least0.60 in the near-infrared spectral range of about 800 nm to about 900nm) and by creating a drift electrical field “E” in the lower region forfast transport of the photoelectrons generated in the secondsemiconductor region 104 by incident NIR light. In practice, a largeportion of the incident NIR light originates from sunlight, while only arelatively small fraction originates from a modulated light source, e.g.a pulsed light source, also referred to herein as “emitter”. Only thelatter portion contains the TOF-information, while the former portion isundesired signal. However, the TOF-information, which is the informationof interest, can be extracted by demodulation techniques.

When exposed to incident radiation, visible range photons are mainlyabsorbed in the first (upper) semiconductor region(s) 101, where visiblelight detectors 108 are located for measuring the amount of chargecarriers (e.g. photoelectrons) created in the first semiconductorregion(s) 101 by absorption of the visible light, as schematicallyindicated by the arrows with label “R” (red) and “G” (green), as anexample only.

NIR range photons are mainly absorbed in the second (lower)semiconductor region 104 located below the isolation layer 102, afterpassing through the first semiconductor region 101 and through theinsulation layer 102, both of which are substantially transparent forthe NIR light, as schematically indicated by the arrow with label “N”.The NIR light creates charge carriers (e.g. photoelectrons) in thesecond (lower) semiconductor region 104, which charge carriers aredetected by one or more detectors, e.g. photodetectors 105 after beingtransported thereto due to the presence of an electrical field Edrift,which is present or created in the second semiconductor region 104 forfacilitating fast transport of said charge carriers.

In order to work fast enough for extracting TOF-information, a lateralelectrical drift field must be present in the second (lower)semiconductor region 104 for transporting the NIR signal to one or moredetection regions 105 (e.g. photodetection regions which may be locatedat the surface) in a sufficiently fast way (mere diffusion is not fastenough). The presence of the electrical field is essential for allowingdetection of the modulated NIR information at a speed (bandwidth)sufficiently high for allowing detection with sufficient accuracy of theemitted NIR signal, after reflection on an object, the distance betweenthe object and the sensor to be measured, and after being added forexample with the sunlight spectrum. The higher the bandwidth, the betterthe distance resolution, e.g. if a TOF camera has 1 cm distanceresolution at 25 MHz, the resolution of the same TOF camera at the sameaverage emitted optical power, using the principles disclosed here,operating at 250 MHz is 1 mm. In embodiments of the present invention,said bandwidth is at least 10 MHz, e.g. at least 25 MHz, e.g. at least50 MHz, e.g. at least 100 MHz, e.g. at least 200 MHz, e.g. at least 300MHz. The electrical field Edrift can be created in several ways, someexamples of which will be described in relation to FIG. 2 to FIG. 4.

It is a major advantage of at least some embodiments of the presentinvention over some prior art (e.g. US2013/0234029) that an insulatinglayer 102 is used to separate the first and the second semiconductorregions 101, 104, wherein the first semiconductor layer 1 and theinsulating layer 2 are substantially transparent to NIR light, becausesuch a structure is easier (and cheaper) to produce than a structurecomposed of two separate substrates which are bonded together. Indeed,such a process requires two wafers which need to be separatelyprocessed, and in order for the top wafer to be bonded to the lowerwafer, it needs to be thinned down to about 3 to about 5 micron, whichtypically requires a third transparent substrate wafer to guaranteemechanical stability. In contrast, even though SOI-wafers are somewhatmore expensive than standard silicon wafers, only a single such wafer isneeded, and no thinning step and no bonding step is required, resultingin an easier process, and in a cheaper device which is also mechanicallymore stable.

By physically arranging the first semiconductor region 101 (alsoreferred to as “color detection region”) on top of the secondsemiconductor region 104 (also referred to as “NIR detection region),the size of both the color detection region(s) and the NIR detectionregion can be increased (as compared to a planar interleaved topology).

The structure shown in FIG. 1 has four image detection regions 101(subpixels), and one NIR detection region 104, but the invention is notlimited thereto, and the first semiconductor region 101 can also havemore than four, or less than four image detection regions 101, forexample only one or only two or only three. When a color filter (e.g. ared or green or blue or any other color) is arranged above or on top ofsaid image detector, each image detector will act as a specific colordetector, for example for sensing red light, green light or blue light.This will be further discussed in relation to FIG. 8 and FIGS.9(a)-9(e). Color filters are known in the art, and can for example bemade from pigmented resin, as is known for example fromUS2013/0214161(A1) and U.S. Pat. No. 5,619,357, both incorporated hereinby reference. Such color filter(s) is/are however not mandatory for thepresent invention, which also works for an image sensor having e.g.grayscale image pixels and TOF pixels. For the sake of explaining FIG.1, it is assumed that an R, G and B color filter are arranged on top ofor above the four image pixels 101 a to 101 d, (101 b and 101 c locatedbetween pixels 101 a and 101 d, but not being indicated not to overloadthe drawing).

Advantageously the four color detection regions 101 a to 101 d arefurther at least partly, but preferably completely electricallyseparated from each other by means of trenches, e.g. DTI trenches (deeptrench isolation). This offers the advantage of reducing electricalcross-talk between adjacent color detectors, because the risk that aphoto-electron generated under e.g. a red subpixel would falsely end-upin the photodetector of e.g. green subpixel is reduced.

It is a major advantage of the insulating layer 102 that it helps toavoid the risk that charge carriers (e.g. electrons or holes) created byNIR photons in the second semiconductor region 103, 104 migrate towardsthe color detectors 108 rather than to the NIR detectors 105, and wouldthus be incorrectly interpreted as color information, and vice versa,that charge carriers created by visible light in the first semiconductorregion(s) 101 would incorrectly migrate towards the NIR photodetectors105, and be incorrectly interpreted as NIR-information. This separationis achieved by the intermediate insulation layer 102, which may forexample comprise a buried oxide layer (BOX). It is an advantage that aburied oxide layer allows passage of NIR light without too muchattenuation. The production of a buried oxide layer and of shallow ordeep trenches is known in the field of SOI-technology.

Detection regions 105 (for NIR light) and 108 (for visible light) arethe means for photo signal detection. It can be implemented e.g. as asimple photodiode, a pinned photodiode or a demodulating detector likePMD or CAPD or any combination thereof. They can be readout by a readoutcircuit such as, for example, a well-known “3T”, “4T” or “TIN-structure.

It is pointed out however that not every detection region needs to haveits own readout circuitry, and it is also possible to use a singlereadout circuit to read-out (e.g. sequentially) multiple detectionregions.

As can be seen in FIG. 1, the detection region 108 of the visible pixelscan be larger than the detector regions 105 of the NIR pixels.

The photodetectors 108 for capturing the charge carriers generated bythe visible light and the photodetectors 105 for capturing the chargecarriers generated by the NIR light, are preferably arranged near thetop surface of the substrate.

The pixel unit 120 may further comprise one or more readout circuits 106adapted for reading out the accumulated charges stored on thephotodetectors 108, 105. In the example of FIG. 1 each photodetectorregion 108 has its own readout circuitry, but that is not absolutelyrequired, and a single readout-circuit 106 may be shared by one or moreor even all the photodetectors 108 of the pixel unit 120, even by theNIR photodetector 105.

The NIR detection regions 105 may have their own dedicated readoutcircuitry (not shown in FIG. 1, but see for example FIG. 8), but that isnot essential for the present invention, and the NIR detector regions105 could also be readout by one of the readout circuits 106 of theimage pixels (by time-sharing), or could be readout by external readoutcircuitry directly connected to the detector region(s) 105. But it isalso possible to readout the demodulated or non-demodulated NIR-signaldirectly, as will be discussed further in relation to FIG. 2 to FIG. 4.

It is an advantage of the structure of FIG. 1 that the visual image data(color data) and the NIR data (e.g. distance data) can be captured(detected, sensed) simultaneously, irrespective of whether the readoutoccurs simultaneously (in parallel) or sequentially. This isadvantageous e.g. for data-consistency, especially when capturing movingobjects.

In a variant of the embodiment shown in FIG. 1, more than four imagepixels or less than four image pixels can be provided in the firstsemiconductor region 101, between pairs of TOF detectors 105, forexample a single, only two or only three image pixels, and each of themmay have its own readout-circuitry 106, or the readout circuitry of someor all of the image pixels may be shared. In a particular embodiment thefirst semiconductor region has exactly three image pixels, for examplefor detecting Red, Green and Blue, and only two readout circuits, one ofwhich readout circuits being arranged for reading out two colordetectors (e.g. Red and Green), the other readout circuit being arrangedfor reading out one color detector (e.g. Blue) and the TOF detector 105.

Finally, it should be mentioned that, although FIG. 1 shows theelectrical field Edrift oriented only in one direction (the arrowspointing from detector 105 on the left to detector 105 on the right),the field Edrift may also be an alternating field, an example of whichwill be described in FIG. 4.

Several ways are possible to generate the lateral electrical fieldEdrift, three examples will be shown in FIG. 2 to FIG. 4, but theinvention is not limited thereto, and other suitable ways of generatinga lateral electrical field underneath the first semiconductor region 101may also be used.

FIG. 2(a) shows a variant of the structure of FIG. 1, wherein the secondsemiconductor region 204 has a non-constant doping profile. FIG. 2(b)shows the substrate doping level 207 taken in a plane A-A. For the caseof SOI processing this doping profile can be approximated, for example,by a step doping 208 of the handle wafer, followed by a high temperatureoperation (drive in) before further SOI processing. This approach can beused for small pitch of the time-of-flight subpixels since thepractically achievable electrical field strength E is rather low(typically in the order of -0.5V/ pixel pitch).

FIG. 2 also shows another feature, which is not intrinsically linkedwith the non-constant doping profile, but is related to a possible wayof reading out the detector 205. The photoelectrons caused by NIR lightmay be detected by a reverse biased diode followed by a demodulatingcircuit, e.g. a mixer 209 (symbolically represented by the circle andX), such as well known analog circuits and techniques (e.g. Gilbertcells or simple switches) or as more sensitive charge domain demodulatordevices, such as majority current assisted photonic demodulator,photonic mixing devices, optical efficiency modulation devices or anyother device with demodulation capabilities.

It is noted that the invention is not limited to the particular dopingprofile shown in FIG. 2(b), and other doping profiles may be used aswell, as long as the maximum doping is somewhere underneath the firstsemiconductor region, e.g. in the middle, and the doping profile ismonotonically decreasing towards an area below the one or more detectors205. Such a doping profile will create an electrical field (indicated bythe arrows) to move photoelectrons towards the detector regions 205 a,205 b, in a direction opposite the arrows.

FIG. 3 shows another variant of the structure of FIG. 1, wherein thesecond semiconductor region 304 is substantially un-doped (i.e.intrinsic), having a first and second detector 305 a, 305 b in the formof a first resp. second contact element. The first contact element 305 ais a n-doped region, and the second contact element 305b is a p-dopedregion, so as to form a P-I-N structure. In this embodiment, the deepelectrical field “E” is created using a lateral p-i-n structurecomprising an n-doped region 305 a, intrinsic or extremely low dopedregion 304, and a p-doped region 305 b. A reverse bias voltage Vbias,applied to this p-i-n structure, creates a lateral electrical driftfield “E” within the second semiconductor region 304. The photoelectronsgenerated in region 304 drift towards the detector region 305 a forcreating a current signal. The n-region together with intrinsic orlightly doped p-substrate creates a NIR-photodetector.

The same demodulation technique, using a modulator 309 as discussed inrelation to FIG. 2 is also shown here, but other ways of readout arealso possible. For example, the photocurrent signal through region 305 amay also be readout and processed in another way (e.g. time domainprocessing, e.g. correlation) by a processing unit (not shown) to obtainthe time-of-flight distance information. And it is also possible toconnect several detector regions 305 of a plurality of pixel units of asemiconductor sensor together to form (or emulate) bigger distancepixels to increase sensitivity, as shown for example in FIG. 6.Demodulation can be also done by modulating a reverse bias of thediodes.

FIG. 4 shows another variant of the structure of FIG. 1, wherein thesecond semiconductor region 404 is a lowly p-doped region. It furthercomprises a first and second detection element, each comprising ann-doped detection region 405 a, 405 b adjacent a p-doped contact region426 a, 426 b. The embodiment shown comprises two p-doped regions 426 a,426 b to create ohmic contacts to the lowly p-doped second region 404,two n-doped detector regions 405 a, 405 b adjacent to the two p-dopedregions. A voltage source 412, connected between the two p-doped regions426 a, 426 b (acting as electrical contacts), creates a majority holecurrent flow inducing a gradual voltage drop over the resistance of thesecond semiconductor region 404, thus creating a deep lateral driftregion 403. The photoelectrons drift towards the p-doped region 426 a or426 b, whichever has a higher potential, once approaching a p-typeregion 426 the photoelectrons are picked up by the electric field of thereversed biased diode formed by n-doped detector regions 405 a, 405 b inthe second semiconductor region 404, which is lightly p-doped.

If the voltage source 412 is a DC voltage, the photocurrent signalthrough detector regions 405 a, 405 b needs to be demodulated orprocessed in another way (e.g. time domain processing) to obtain thetime-of-flight distance information. If the voltage source 412 is avoltage having a modulated waveform synchronous to the near-infraredactive illumination signal (which may e.g. be a modulated or pulsed LEDsignal), the demodulation can be done in the second semiconductor region404 itself: the modulated voltage source 412 will then create a varyingelectric field Efield in the deep lateral drift region 403 decomposingthe photocurrent into an in-phase component and an out-of-phasecomponents collected separately by the detector regions 405 a, 405 b.

In all the examples shown in FIG. 1 to FIG. 4, the substrate canoptionally be biased by a negative voltage source “−Vsub” (with respectto the voltages used to create the drift field E) to provide anadditional vertical electric field component (not shown) which mayfurther improve the speed and sensitivity of the time-of-flight distancesensor. It is noted that that this bias voltage Vsub is different fromthe bias voltage Voxbias which will be described with reference to FIG.12 to FIG. 17. Both voltages Vsub, Voxbias may be present, and they canbe chosen independently.

As explained above, the main purpose of the first semiconductor regions101, 201, 301, 401 is detection of an image signal (i.e. visible lightsignal, e.g. color or brightness information) and absorption of (mostof) the visible range of the spectrum of the impinging light signal. Theoperation of the subpixels (e.g. photodetectors) within the firstsemiconductor regions 101, 201, 301, 401 is the same as the operation ofconventional image sensor pixels since the electric field created in thesecond semiconductor regions 104, 204, 304, 404, e.g. the deep lateraldrift regions 103, 203, 303, 403, is decoupled from the image sensorsubpixels by the insulation medium 102, 202, 302, 402. The image sensorsubpixels can be implemented, for example, as 3T or 4T pixels withpinned photodiodes.

The sunlight is a required signal for obtaining visible information(e.g. color information), but is an undesirable signal for thetime-of-flight distance sensor since it degrades the signal to noiseratio SNR and may cause a saturation of the signal readout circuits. Bychoosing an appropriate thickness, the first semiconductor region 101,201, 301, 401 absorbs the major part of the visible sunlight spectrumwhile letting the near-infrared signal pass through to the secondsemiconductor regions 104, 204, 304, 404 where the deeply generatedphotoelectrons are transported by the lateral electrical field “E” tothe detector regions 105, 205, 305, 405.

FIG. 5 shows a typical visible sunlight spectrum (at sea level) depictedin curve 501, incident to a semiconductor device. After passing througha 5.0 micron thick first semiconductor region, the sunlight spectrum isattenuated resulting in the curve 502. If the pixel unit, orsemiconductor sensor device is covered with a color filter (array) toform a color image sensor, the sunlight power 503 reaching the secondsemiconductor region is further reduced, since each image sensor pixelcolor filter is passing only a limited part of the visible sunlightspectrum, e.g. only a blue, green or red part. The average sunlightspectral power after passing through the color filter array and thefirst semiconductor region is depicted by the curve 503. The totalsunlight power (integrated over the visible wavelength range) thatreaches the second semiconductor region (area of the hatched regionunder curve 503) is typically less than 5% of the visible sunlight powerimpinging on the chip. At the same time, the near-infrared lightattenuation is not significant, as almost 90% of the 900 nmnear-infrared light is passing through the first semiconductor region(assuming the color filter array is substantially transparent tonear-infrared light), as will be further discussed in relation to FIGS.9(a)-9(e).

Here above, a semiconductor pixel unit is described, capable ofsimultaneously sensing image information and distance information withan improved sensitivity as compared to the prior art, and/or which canbe produced in an simpler and cheaper manner. This pixel unit can beembodied in a two-dimensional array for forming a 3D image sensor.Several topologies can be chosen with regards to e.g. the number andkind of the image subpixels in the first semiconductor region. Twoexamples are described below, in relation to FIG. 6 and FIG. 7, but theinvention is not limited to these examples, and will also work withother topologies.

FIG. 6 shows a top view of a possible arrangement of a sensor deviceaccording to the present invention comprising a plurality or array ofpixel units 720 (indicated in dashed line) according to the presentinvention. If the drawing of FIG. 1 would have two image subpixels(instead of four) in the first semiconductor region 101, it could beseen as a cross section of the pixel unit 720 of FIG. 6 according to theline X-X.

In the example shown in FIG. 6, each pixel unit has four imagesubpixels, e.g. organized in a Bayer-like pattern, each group having onesmall distance subpixel (indicated with reference T), each distancesubpixel comprising a contact 704 to the second semiconductor regionwhich is located underneath the four color subpixels, a small n-dopeddetection region and its own dedicated readout circuit. Of course, thepresent invention is not limited to this particular RGGB-color-pattern,and other color patterns may also be used. In the example shown in FIG.6, the distance subpixel 702 may have no dedicated readout circuit,hence the readout circuit of one of the four color pixels can be usedfor reading out the information of the distance subpixel.

FIG. 6 can also be seen as a sensor array comprising the combination of:

-   a two-dimensional array of image subpixels 720 located in first    semiconductor regions, each first semiconductor region comprising    four image subpixels, and-   a two-dimensional array of time-of-flight distance subpixels 702    located in a second semiconductor region, as described above, each    distance pixel comprising approximately the same area as the four    image sub-pixels.

A deep lateral drift region is created in the second regions underneaththe first regions by any of the means described above in relation toFIG. 1 to FIG. 4, preferably the majority current assisted drift fieldinduction method shown in FIG. 4, since it allows easy electric fieldmodulation over extended areas. The image sensor array operates in aconventional mode, in the sense that visible light createsphotoelectrons in the first semiconductor regions, which photoelectronsare accumulated on color detectors, and create a charge which is read bymeans of a classical readout circuit. The photoelectrons that aregenerated deeply in the second semiconductor region (by thenear-infrared photons) are drifting fast towards the TOF detectors (alsoreferred to as time-of-flight distance subpixels). The time-of-flightdistance sensor subpixels 702 comprise means for establishing a deeplateral drift region, a detection region for measuring the photocurrentinduced in the second semiconductor region. There is no need of thelarge sensing diode for the time-of-flight pixels so they can be madesmaller than image pixel sensors without a significant loss ofsensitivity. The readout of the time-of-flight signal can be done usinga dedicated readout circuit (e.g. 3T or 4T) or the readout circuit canbe shared with one of the adjacent image sensor pixel. If the deeplateral drift field is constant the time-of-flight distance sensorsubpixels 702 may include means for time-of-flight estimation such ase.g. a demodulation device or a time-domain signal processing block. Thedemodulation can also be done outside of the array by connecting adistance subpixel or shorted group of distance subpixels to an externaldemodulating circuit.

FIG. 7 shows a top view of another possible arrangement of a sensorarray comprising multiple pixel units 620 according to an embodiment ofthe present invention. Each pixel unit 620 of the sensor device shown inFIG. 7 comprises 5×15=75 visual light detectors and acts as a singledistance detector, but operates in the same manner as described for thesensor device of FIG. 6.

Also this sensor device 600 comprises an array of image sensor pixels601 located in the first semiconductor region, an array oftime-of-flight distance sensor subpixels 605 located in the secondsemiconductor region which is electrically isolated from the firstsemiconductor region by an insulating medium transparent to NIR light. Amodulated deep lateral drift region is induced in the secondsemiconductor region by a modulated voltage source.

The time-of-flight distance sensor subpixels 605 (NIR detectors) of thepreferred embodiment comprise means for establishing a modulated deeplateral drift region, and a detection region for measuring thephotocurrent induced in the second semiconductor region. The preferredembodiment utilizes a majority hole current assisted method of themodulated lateral drift field induction via ohmic contacts to the secondsemiconductor region. A time-of-flight photo charge signal is deliveredto the detection region by the induced deep lateral drift field fromunderneath the imaging pixels.

This sensor embodiment fits well many real world applications that donot require a high spatial resolution of the distance information, forinstance, an automotive side view camera with a blind spot monitoring.

FIG. 8 shows a particular embodiment of a semiconductor pixel unit 820according to the present invention. In this embodiment, the firstsemiconductor region contains two image pixels 801 a, 801 b, each ofwhich has its own readout circuit 806 a, 806 b. In the embodiment shownin FIG. 8, the image pixels 801 a, 801 b are electrically isolated fromone another by means of a deep trench 801 b, but the presence of thistrench 801 b is not mandatory, and even if present, need not extend tothe burned oxide layer 102. The presence of the trenches 810 a and 810 chowever is important to prevent photoelectrons generated in one regionending up in the other region. The NIR detector 805 a has about the samesize as those of the image detector 801, and it also has its own readoutcircuit 806 n. Furthermore, there are two ohmic contact regions forapplying a voltage for creating the lateral drift field in the secondsemiconductor region 804.

This embodiment is also used to illustrate another aspect, which canalso be used in any of the previously described embodiments. As shown, afirst color filter 831a, with label RED-N, e.g. having an opticalcharacteristic like the one shown in FIG. 9(e), is applied above or ontop of a first image pixel 801 a, which is therefore an image pixeladapted for capturing red light components, and is therefore called a“red color pixel”. A second color filter 831 b, with label BLUE-N, e.g.having an optical characteristic like the one shown in FIG. 9(c), isapplied above or on top of a second image pixel 801 b, which istherefore an image pixel adapted for capturing blue light components,and is therefore called a “blue color pixel”.

Most of the Red incident light will pass through the RED-N filter 831 a,and be absorbed in the first semiconductor region 801 a. Most of theGreen and Blue light will be blocked by the RED-N filter 831 a. Most ofthe NIR light will pass through the RED-N filter 831 a, and through thefirst semiconductor region 801 a, and through the insulating layer 802,and be absorbed in the second semiconductor region 804, or part 803thereof where the deep lateral drift field E is present.

The embodiment of FIG. 8 also has microlenses 840 arranged above each ofthe image detectors and above the NIR detector. Microlenses can also beadded to the embodiments of FIG. 1 to FIG. 4, irrespective of thepresence or absence of color filters 831. It is noted that the presenceof microlenses is completely optional, but when present, it may help toimprove the sensitivity of the visible and NIR pixels.

Finally, the embodiment of FIG. 8 also has a so called “short-pass andbandpass” filter 850, which will be described in relation to FIGS.9(a)-9(e). Also this filter 850 is purely optional, and can be presentin any of the embodiments shown in FIG. 1 to FIG. 4, irrespective of thepresence or absence of the microlenses 840 and/or color filters 831.

FIGS. 9(a)-9(e) show a set of suitable filters as may optionally beadded to semiconductor pixel units of the present invention, to furtherimprove the performance or accuracy. The relevant part of the spectrumis the visible light spectrum and a part of the NIR spectrum, forexample the part of the spectrum from 390 nm to about 900 nm, wherein itis assumed that the light by the light source for measuringTOF-information is a light source that creates a signal having a narrowspectrum centered around 850 nm, as shown for example in FIG. 9(a).

FIG. 9(b) shows an optical filter (herein referred to as “short pass +bandpass filter”) (with a characteristic 907) for blocking light havingwavelengths from about 750 nm to about 800 nm and wavelengths higherthan about 900 nm, while passing light having wavelengths in the visiblerange from about 390 nm to about 750 nm and in a relatively narrow bandin the NIR range, for example from about 800 nm to about 900 nm. Such afilter can be realized in known ways, e.g. an “interference filter” canbe used to achieve the desired response. Such filters may be composed ofseveral layers of materials with different refraction index. This filtertechnology allows an arbitrary transmission characteristic. This filteris advantageously located over or in front of the entire substrate area.Interference type filters are known in the art, for example as describedin FR2904432, incorporated herein by reference.

FIG. 9(c) shows a blue color filter (with a characteristic 901) as canadvantageously be used on top of some of the image pixels. This colorfilter allows passage of blue light, while blocking (or at leastattenuating) green and red light, and allows passage of NIR light in thepredefined range of about 800 to about 900 nm.

FIG. 9(d) shows a green color filter (with a characteristic 905) as canadvantageously be used on top of some of the image pixels. This colorfilter allows passage of green light, while blocking (or at leastattenuating) blue and red light, and allows passage of NIR light in thepredefined range of about 800 to about 900 nm.

FIG. 9(e) shows a red color filter (with a characteristic 906) as canadvantageously be used on top of some of the image pixels. This colorfilter allows passage of red light, while blocking (or at leastattenuating) blue and green light, and allows passage of NIR light inthe predefined range of about 800 to about 900 nm.

FIG. 9(a) shows all these curves in a single graph.

Color filters such as the ones shown in FIG. 9(c) to FIG. 9(e) can beused to pass only a single color component to a particular image pixel,so that it acts as a color pixel. This behavior is known from digitalimage cameras, except that for the present application, it is importantthat the filter allows passage of part of the NIR spectrum, e.g. fromabout 800 to about 900 nm. It is advantageous that the visible lightfilters (for example red, green or blue color filters) are at leastpartially, preferably as much as possible transparent for NIRwavelengths, so that almost the entire space of the pixel unit under thecolor detectors, or more accurately under the insulation layer 102, canbe used for capturing NIR light. In practice of course, 100% passage and100% blockage is not possible, but materials having an optical filtercharacteristic resembling the curves shown in FIG. 9(c) to FIG. 9(e) doexist, and are known in the art.

The behavior of the filter 850 having the characteristic 907 shown inFIG. 9(b) for wavelengths lower than 390 nm (i.e. UV light) is notimportant when this filter is used in combination with the color filtersshown in FIG. 9(c) to FIG. 9(e), because the color filters block suchwavelengths already. However, the filter 850 is preferably designed toblock light having a wavelength above 900 nm, as much as possible.

As mentioned before, the presence of the microlenses 840, and/or thecolor filters 831, and/or the so called “short pass+bandpass filter” 850is not essential for the present invention, but when present, the imagesensor can capture color images, and the accuracy of the TOF-measurementcan be increased. In preferred embodiments of the present invention,such microlenses 840 and color filters 831 and such “short pass +bandpass filter” 850 are present. Providing microlenses 840 isadvantageous because it focuses incident light on the metallization-freephoto sensitive silicon area. This further helps to reduce, e.g.minimize optical cross talk, which is beneficial for the image quality.

FIG. 10 shows (in dotted line) the optical transmission characteristicof a silicon film having a thickness of 5.0 micron resp. of 3.5 micronfor incident radiation having a wavelength in the range of about 400 toabout 1000 nm. The shown dip from about 750 nm to about 800 nm is due tothe filter shown in FIG. 9(b), when present. The purpose of this graphis to show that a major portion (the hashed area above the transmissioncurve from 400 to 750 nm) of the visible light is blocked by the first(upper) semiconductor layer before reaching the second semiconductorregion. Also a small portion of the NIR light will be blocked (the areaabove the transmission curve from 800 to 900 nm), but a major portion(the hashed area below said curve) is passed to the second semiconductorlayer. Thus the NIR part has very small attenuation when passing throughthe first (upper) semiconductor region, while the visual light is mostlyabsorbed (especially blue and green light) before reaching the second(lower) semiconductor region.

If other semiconductor material were used, e.g. Germanium, or aSi-compound or a Ge-compound, then of course a slightly differentcharacteristic would be obtained, but the same principles would stillapply: the first semiconductor region absorbs most of the visible lightwhile passing NIR light. The skilled person can choose a suitablethickness of the first and second semiconductor layer depending on theoptical characteristics, in particular the absorption characteristic ofvisible light of the material being used.

FIG. 11 shows the typical sunlight spectrum of FIG. 5 in combinationwith the “short-pass+bandpass” filter of FIG. 9(b), if present. It showsthe power spectrum of the radiation reaching the second (lower)semiconductor region, as the hashed areas.

The purpose of the filters (if present) is to avoid or at least reducethe pollution of the TOF NIR signal by the sunlight shot noise. Thesunlight reaching the bottom layer can thus be filtered by severalstages: (1) by the “short-pass+bandpass” filter as shown e.g. in FIG.9(b): it rejects everything except the visible light and a narrow TOFband, (2) by the color filters as shown e.g. in FIG. 9(c) to FIG. 9(e):they pass the color+NIR, (3) by the top silicon layer as shown e.g. inFIG. 10 (dotted curve, without the dip from 750 to 800 nm).

Performance:

The above part of the description describes mainly the three-dimensionalstructure and the effect of the lateral electrical field. It was found,however, that the performance of the device, in particular the speed ofthe NIR light detection and demodulation, may be negatively influencedby the presence of positive charge inside the insulation layer, e.g.positive charge trapped in the oxide, or near the oxide/siliconinterface.

It seems to be inevitable in semiconductor processing that at least someamount of positive charge ends up in silicon-oxide. While this does notcause a performance problem (or limitation) for devices such as forexample MOSFETs, because such devices typically use heavily dopedsilicon (e.g. about 10¹⁷/cm³), if no measure is taken, it typically willcause create a performance problem (or limitation) for devices accordingto the present invention, using intrinsic or lowly doped (e.g. about10¹²/cm³) semiconductor materials, which is e.g. 5 orders of magnitudelower. This phenomenon will typically occur in all the devices shown inFIG. 1 to FIG. 8, but once realized and understood, can be solved aswill be described next, with reference to FIG. 12 to FIG. 16.

FIG. 12 shows the structure of FIG. 1, further provided with means forapplying a DC bias voltage referred to as ‘Voxbias’ between the firstsemiconductor region 1201 and the second semiconductor region 1204, asan example. This may be implemented in practice by connecting the secondsemiconductor region to ground, and by connecting the firstsemiconductor region to a voltage source relative to ground (as is shownin FIG. 12). This DC bias voltage ‘Voxbias’ can also be applied to anyof the structures of FIG. 2 to FIG. 8, and can be chosen independentlyfor example from the bias voltage ‘Vbias’ of FIG. 3 (which creates thelateral electrical field), and from the voltage Vmod of FIG. 4 (whichcreates the lateral electrical field).

FIG. 13(a) shows a conduction band diagram for an electrically neutraloxide for the structure of FIG. 4 at different positions on the thickblack dotted line of FIG. 12, directly below the oxide, at a particularmoment in time, at which a voltage V1 is applied to the p+region shownon the right of FIG. 13(a) and a voltage V2 higher than V1 is applied tothe p+ region shown on the left of FIG. 13(a). The p+ region may forexample have a doping level in the range of 10¹⁵ to 10¹⁹/CM³, forexample about 10¹⁶/cm³, for example about 10¹⁷/cm³, for example about10¹⁸/cm³.

FIG. 13(b) shows a conduction band diagram for the same locations, incase a positive charge is trapped inside the oxide. This positive chargewill attract photoelectrons generated in the second semiconductor regionto move upwards towards the oxide-interface.

The speed performance of the demodulating sensor depends on the driftelectrical field strength and electrostatic potential monotonicity fromthe point where the photocharge is generated (e.g. somewhere inside thedeep lateral drift region) to the point where it is detected in thedetection region (e.g. one of the n+ regions at the surface). If at somepoint in the region 104 the electrostatic potential is not monotonous, acharge trap forms, which can significantly degrade the demodulatingspeed of the device.

The oxide contains positive interface charge and typically there is anadditional positive charge trapped in the oxide itself from the processof oxide growth. The structure depicted in FIG. 13(a) is a simplifiedversion of an “RGBZ pixel”, in particular a small “slice” thereof justabove and just below the interface between the oxide and the secondsemiconductor region (indicated by the bracket in FIG. 12). Ademodulating drift field is created by applying an alternating voltageV1(t) and V2(t) to the p+ regions 1205 a and 1205 b. The energy banddiagram is sampled along a horizontal line located beneath the oxidealong the oxide—semiconductor interface (the thick dotted line of FIG.12). The positive charge present in the oxide shifts the energy band inthe underlying semiconductor along the oxide-semiconductor interfacecausing a formation of a “charge trap” which deteriorates the speed ofthe sensor. For example, at least some of the photoelectrons which aresupposed to be detected by the left n+ detection region 1305 a gettrapped in the charge pocket, which needs to be filled before they orfurther photoelectrons can continue to flow on to the n+ detectionregion 1305 a. The trapped photoelectrons leave the charge trap slowlyby diffusion. The trapped photoelectrons can even be detected by thewrong detection node if they remain trapped at the time when thedemodulating electrical field direction is reversed. These effects mayhave a significant impact on the speed performance and on the devicelinearity.

The impact of the bias voltage ‘Voxbias’ applied between the first andsecond semiconductor region and thus over the oxide (in case theinsulation region is an oxide) on the speed of the device can forexample be seen from the experimental measurement plot of “demodulationcontrast” versus “modulation frequency” of FIG. 14.

This measurement was taken on an experimental test structure verysimilar to the structure shown in FIG. 12, having a size “S” of about 7micron, where the first semiconductor region was made of poly-silicon,and was used as an electrode on top of the oxide region. Thepoly-silicon electrode is at least transparent in the NIR wavelengthrange. As can be seen, the “demodulation contrast” (speed) in thetypical operating frequency range, e.g. the range from 30 to 50 MHz, israther low (e.g. lower than 0.4) for Voxbias equal to −2V and 0Vrespectively. However, for more negative Voxbias voltage, in thisstructure for example for −4V, the demodulation contrast is acceptablyhigh (e.g. higher than 0.6) for frequencies up to about 150 MHz.

FIG. 15 shows a plot similar to FIG. 14 but for another device producedwithout a “transparent electrode” on top of the oxide, (i.e. without thefirst semiconductor region, meaning that there was only an oxide on topof the second semiconductor region), and having a size S (see FIG. 12)of about 14 micron. The electric drift field was created under the oxideby means of two ohmic contacts with a modulated voltage source connectedbetween them. In this example, the demodulation contrast was equal toabout 20% for modulation frequencies in the range of 20 to 50 MHz. Nobias voltage was applied over the oxide.

Referring back to FIG. 14, it was found that the optimal (e.g. in termsof maximum speed) Voxbias voltage is slightly more negative than thevoltage required to achieve a flat band condition (e.g. about 100 mVmore negative). Using such bias voltage, the photoelectrons generated byNIR light in the second semiconductor region are pushed away from theoxide surface, so that the probability of recombination or trapping isreduced. If the Voxbias voltage is too negative, an accumulation layerin the semiconductor can occur, leading to a much increased currentconsumption, thus power consumption. For any given structure and for agiven modulation frequency (of the light source), the skilled person canfind a suitable Voxbias voltage by experimentation (if the structure isadapted for applying such a voltage, of course).

In one embodiment, the applied Voxbias voltage is a constant predefinedDC voltage. This can for example be used if the the amount of positivecharge trapped in the oxide can be controlled within predefined margins(during production).

In another embodiment, a control loop (e.g. regulating the currentconsumption through the bias electrodes 426 by setting the Voxbiasvoltage) may be established to achieve the optimal voltage for Voxbiaswhich will maximize the speed of the device while preventing theexcessive power consumption due to the formation of the accumulationlayer.

In yet another embodiment, the optimal bias voltage Voxbias isdetermined by means of a calibration step.

Quite surprisingly, the biasing voltage Voxbias allows the modulationfrequency to be increased much higher than the typical 30 to 50 MHzfrequency, while still providing a demodulation contrast of at least45%. The maximum obtainable value depends on mainly 3 parameters:

-   -   (1) the smaller the dimensions of the device, the shorter the        distance the NIR photoelectrons need to travel, the faster the        device,    -   (2) the larger the magnitude of the modulating signal, e.g. the        amplitude of the modulating voltage signal, the higher the        lateral electrical drift field, the faster the device, and    -   (3) the bias voltage Voxbias applied over the oxide.

In particular embodiments of the present invention, the dimensions ofthe device (in particular the size S of FIG. 12) and the bias voltageVoxbias, and the amplitude of the modulation signal are chosen suchthat, e.g. at room temperature:

-   -   a) the demodulation contrast is at least 65% and the modulation        frequency is at least 100 MHz, or    -   b) the demodulation contrast is at least 60% and the modulation        frequency is at least 100 MHz, or    -   c) the demodulation contrast is at least 55% and the modulation        frequency is at least 100 MHz, or    -   d) the demodulation contrast is at least 50% and the modulation        frequency is at least 100 MHz, or    -   e) the demodulation contrast is at least 50% and the modulation        frequency is at least 200 MHz, or    -   f) the demodulation contrast is at least 40% and the modulation        frequency is at least 300 MHz.

Similar considerations also apply to p-i-n detectors formed in theregion 104, as shown in the structure of FIG. 3. Also here, a biasvoltage Voxbias can be applied between the first semiconductor region301 and the second semiconductor region 304 to create a verticalelectrical field component over the horizontal part of the buried-oxidelayer BOX 302.

FIG. 16 illustrates a typical qualitative improvement when applying thebias voltage Voxbias between the first and second semiconductor region.The plot shows “demodulation contrast” versus the bias voltage ‘Voxbias’for a given modulation frequency (for example about 20 MHz). Althoughthe absolute values depend on the particular structure (dimensions andmaterials) being used, it can roughly be stated that for relatively lowmodulation frequencies (e.g. in the range of 20 to 50 MHz) thedemodulation contrast can typically be increased from below 40% (when nobias voltage Voxbias is applied) to at least 60%, by applying a suitablebias voltage Voxbias. And that for relatively high modulationfrequencies (e.g. higher than 100 MHz or even higher than 200 MHz), thedemodulation contrast can typically be increased from below 20% (when nobias voltage Voxbias is applied) to at least 50%, by applying a suitablebias voltage Voxbias.

In FIG. 12 a semiconductor structure was described, wherein the firstsemiconductor region has detection regions and readout circuitry fordetecting visible light information. In FIG. 17 a variant of this pixelunit is shown, not having such detection regions and readout circuitryfor detecting visible light information, but only detection regions 1705for detection of NIR light. Apart from this difference, most of what hasbeen described above, for the embodiments of FIG. 1 to FIG. 12 is alsoapplicable for the embodiment of FIG. 17. Stated differently, if thecircuitry for detecting and/or reading out visible light information isomitted from the embodiments shown in FIG. 1 to FIG. 11, one wouldobtain variants of FIG. 17. Everything described above in relation toNIR detection in the embodiments of FIG. 1 to FIG. 16 is also valid forthese variants of FIG. 17, except of course that no visible light isbeing captured. In these embodiments, the color filters of FIG. 8 may beomitted, and the filter characteristics of the so called “shortpass+bandpass filter” 850, in particular in the visible spectrum rangefrom 390 to 750 nm is less relevant. In fact, in these embodiments (nothaving visible light detectors) a bandpass filter adapted to pass lightonly in the range from about 800 nm to about 900 nm could also be used,and may actually provide even better results.

It is explicitly pointed out that not all claimed embodiments of thepresent invention are explicitly shown in the figures, since mostdrawings are intended to illustrate only particular features. Otherembodiments of the invention are recited by the claims, and theirdependencies.

For example, all embodiments of the present invention have in common: asingle substrate with a first and second semiconductor regionelectrically separated by an insulating region (preferably an oxide),and means for generating a lateral electrical field below the firstsemiconductor region.

But the following features can for example vary amongst the differentembodiments:

-   -   means for generating lateral electrical field: (a) non-constant        doping profile (as illustrated in FIG. 2), (b) intrinsic doping        profile and PIN-structure (as illustrated in FIG. 3), (c) lowly        doped profile and p-plus regions and n-plus regions (as        illustrated in FIG. 4),    -   first semiconductor region with / without detection regions        and/or readout circuitry for capturing and/or readout of visible        light information,    -   insulating layer: (a) buried oxide with optional trenches, (b)        SOT, (c) nitride layer,    -   insulating layer shape: e.g. bathtub, flat or planar;    -   first semiconductor region material: e.g. monocrystalline        silicon or polysilicon;    -   first semiconductor region is e.g. continuous or segmented,        placed only over the critical areas, where the charge traps can        form;    -   first semiconductor region negatively biased w.r.t. second        semiconductor region: (a) yes, (b) no,    -   any of microlenses, color filter, shortpass+bandpass filter or        other filter present: (a) yes, (b) no.

1. A semiconductor pixel unit adapted for sensing near-infrared light,the semiconductor pixel unit comprising: a single semiconductorsubstrate comprising a first semiconductor region and a secondsemiconductor region electrically separated from the first semiconductorregion by means of an insulating region; a doping profile configured togenerate a lateral electrical field in a region underneath the firstsemiconductor region, the electrical field being adapted for promotingtransport of photoelectrons in the second semiconductor region generatedby the near-infrared light after passing through the first semiconductorregion and through the insulating region; the second semiconductorregion comprising at least one near infrared light detector located atthe surface of the semiconductor substrate for detecting thephotoelectrons generated in the second semiconductor region by the nearinfrared light, wherein the insulating region between the firstsemiconductor region and the second semiconductor region comprises aburied oxide layer.
 2. The semiconductor pixel unit according to claim1, further adapted for simultaneously sensing visible light andnear-infrared light, wherein the first semiconductor region comprises atleast one visible light detector for detecting photoelectrons generatedin the first semiconductor region by the visible light.
 3. Thesemiconductor pixel unit according to claim 1, wherein the firstsemiconductor region is arranged to absorb visible light.
 4. Thesemiconductor pixel unit according to claim 1, further comprising avoltage source configured for negatively biasing the first semiconductorregion with respect to the second semiconductor region to affect theenergy bands near the surface of the second semiconductor region over atleast a portion of the insulation region so that the photoelectronsgenerated in the second semiconductor region by the near-infrared lightare pushed away from the insulating region.
 5. The semiconductor pixelunit according to claim 1, wherein the buried oxide layer comprises ahorizontal portion parallel to the substrate surface.
 6. Thesemiconductor pixel unit according to claim 1, wherein the insulatingregion between the first semiconductor region and the secondsemiconductor region further comprises at least one insulating trench,preferably extending from the semiconductor surface to the insulatinglayer.
 7. The semiconductor pixel unit according to claim 1, wherein thefirst semiconductor region comprises at least two visible lightdetectors laterally separated by at least one electrically insulatingtrench, the trench preferably extending from the semiconductor surfaceto the insulating region.
 8. The semiconductor pixel unit according toclaim 1, wherein the second semiconductor region has a uniform dopingprofile as the doping profile configured to generate said lateralelectrical field.
 9. The semiconductor pixel unit according to claim 1,wherein either the second semiconductor region has a non-constant dopingprofile as the doping profile configured to generate said lateralelectrical field, or the second semiconductor region has an intrinsicdoping level, and wherein the semiconductor pixel unit has a firstdetector element being an n-doped region and a second detector elementbeing a p-doped region, so as to form a P-I-N structure as the dopingprofile.
 10. The semiconductor pixel unit according to claim 1, whereinthe second semiconductor region is a lowly p-doped region, and whereinthe semiconductor pixel unit has a first n-doped detector element and asecond n-doped detector element, and further comprising a first p-dopedcontact region arranged in the second semiconductor region adjacent thefirst detector element, and a second p-doped contact region arranged inthe second semiconductor region adjacent the second detector element,the first and the second contact regions allowing a voltage to beapplied for creating said lateral electrical field.
 11. Thesemiconductor pixel unit according to claim 1, further comprising atleast one readout circuit for reading out data of the near infraredlight detector and/or further comprising at least one readout circuitfor reading out data of the visible light detector.
 12. Thesemiconductor pixel unit according to claim 1, further comprising ademodulator having a first input connected to the NIR detector and asecond input for receiving a modulation waveform from a modulator, andhaving an output for providing a demodulated NIR signal to a distancedetermination circuit.
 13. The semiconductor pixel unit according toclaim 12, wherein said demodulator is adapted for operating at ademodulation frequency of at least 10 MHz, and wherein saidsemiconductor pixel unit further comprises a predefined DC voltagesource for providing a bias voltage, the bias voltage being chosen suchthat the demodulation contrast of the pixel unit is at least 50%.
 14. Asemiconductor pixel unit comprising: a single semiconductor substratecomprising a first semiconductor region and a second semiconductorregion electrically separated from the first semiconductor region bymeans of an insulating region; a doping profile configured to generate alateral electrical field in a region underneath the first semiconductorregion, the electrical field being adapted for promoting transport ofphotoelectrons in the second semiconductor region generated by thenear-infrared light after passing through the first semiconductor regionand through the insulating region; the second semiconductor regioncomprising at least one near infrared light detector located at thesurface of the semiconductor substrate for detecting the photoelectronsgenerated in the second semiconductor region by the near infrared light,wherein the semiconductor pixel unit further comprises at least onemicrolens arranged on top of said at least one visible light detector,and/or further comprising at least one color filter arranged on top ofsaid at least one visible light detector, the color filter having acharacteristic for passing visible light in a first predefined band ofspectrum, and for blocking or at least attenuating visible light in asecond predefined band of the spectrum, and for passing a predefinedband of near infrared light, and/or further comprising an optical filterhaving a characteristic for passing visible light in a first band, andfor blocking or at least attenuating light in a second band from about750 nm to a predefined first frequency and for passing near-infraredlight in a third band from the predefined first frequency to apredefined second frequency, whereby the predefined first frequency is avalue in the range of 750 nm to a wavelength below the maximumwavelength of semiconductor sensitivity and the predefined secondfrequency is a value in the range from the predefined first frequency tothe maximum wavelength of semiconductor sensitivity.
 15. A semiconductorsensor comprising: a sensor array comprising a plurality ofsemiconductor pixel units according to claim 12; a distancedetermination circuit for deriving distance information based on theemitted near-infrared light and the measured reflected near-infraredlight.
 16. The semiconductor sensor according to claim 15, furthercomprising: the modulator adapted for operating at a predefinedfrequency of at least 10 MHz, and a demodulator, and a predefined DCvoltage source for providing a bias voltage between the first and secondsemiconductor region, the bias voltage preferably being chosen such thatthe demodulation contrast of the pixel unit is at least 50%.
 17. Adigital camera comprising the semiconductor sensor of claim 16, andfurther comprising: a light source for emitting a modulated or pulsednear-infrared light to be reflected against objects in a scene to becaptured; circuitry for transmitting modulated infrared or near-infraredlight, and circuitry for demodulating the received near-infrared lightand for converting it into distance information.
 18. A semiconductorpixel unit adapted for sensing near-infrared light and visible light,the semiconductor pixel unit comprising: a single semiconductorsubstrate comprising a first semiconductor region and a secondsemiconductor region electrically separated from the first semiconductorregion by means of an insulating region; the first semiconductor regioncomprising at least one visible light detector for detectingphotoelectrons generated in the first semiconductor region by thevisible light, a doping profile configured to generate a lateralelectrical field in a region underneath the first semiconductor region,the electrical field being adapted for promoting transport ofphotoelectrons generated in the second semiconductor region by thenear-infrared light after passing through the first semiconductor regionand through the insulating region, the second semiconductor regioncomprising at least one near infrared light detector located at thesurface of the semiconductor substrate for detecting the photoelectronsgenerated in the second semiconductor region by the near infrared light,wherein the insulating region between the first semiconductor region andthe second semiconductor region comprises a buried oxide layer.
 19. Thesemiconductor pixel unit according to claim 18, further comprising avoltage source configured for negatively biasing the first semiconductorregion with respect to the second semiconductor region to affect theenergy bands near the surface of the second semiconductor region over atleast a portion of the insulation region so that the photoelectronsgenerated in the second semiconductor region by the near-infrared lightare pushed away from the insulating region.